Sex and litter effects on anxiety and DNA methylation levels of stress and neurotrophin genes in adolescent rats

Chemogenetic inhibition of central amygdala CRF-expressing neurons decreases alcohol intake but not trauma-related behaviors in a rat model of post-traumatic stress and alcohol use disorder

Post-traumatic stress disorder (PTSD) and alcohol use disorder (AUD) are often comorbid. Few treatments exist to reduce comorbid PTSD/AUD. Elucidating the mechanisms underlying their comorbidity could reveal new avenues for therapy. Here, we employed a model of comorbid PTSD/AUD, in which rats were subjected to a stressful shock in a familiar context followed by alcohol drinking. We then examined fear overgeneralization and irritability in these rats. Familiar context stress elevated drinking, increased fear overgeneralization, increased alcohol-related aggressive signs, and elevated peripheral stress hormones. We then examined transcripts of stress- and fear-relevant genes in the central amygdala (CeA), a locus that regulates stress-mediated alcohol drinking. Compared with unstressed rats, stressed rats exhibited increases in CeA transcripts for Crh and Fkbp5 and decreases in transcripts for Bdnf and Il18. Levels of Nr3c1 mRNA, which encodes the glucocorticoid receptor, increased in stressed males but decreased in stressed females. Transcripts of Il18 binding protein (Il18bp), Glp-1r, and genes associated with calcitonin gene-related peptide signaling (Calca, Ramp1, Crlr-1, and Iapp) were unaltered. Crh, but not Crhr1, mRNA was increased by stress; thus, we tested whether inhibiting CeA neurons that express corticotropin-releasing factor (CRF) suppress PTSD/AUD-like behaviors. We used Crh-Cre rats that had received a Cre-dependent vector encoding hM4D(Gi), an inhibitory Designer Receptors Exclusively Activated by Designer Drugs. Chemogenetic inhibition of CeA CRF neurons reduced alcohol intake but not fear overgeneralization or irritability-like behaviors. Our findings suggest that CeA CRF modulates PTSD/AUD comorbidity, and inhibiting CRF neural activity is primarily associated with reducing alcohol drinking but not trauma-related behaviors that are associated with PTSD/AUD.

The role of epigenetics in anxiety disorders

Anxiety disorders (ADs) are extremely common psychiatric conditions that frequently co-occur with other physical and mental disorders. The pathophysiology of ADs is multifaceted and involves intricate connections among biological elements, environmental stimuli, and psychological mechanisms. Recent discoveries have highlighted the significance of epigenetics in bridging the gap between multiple risk factors that contribute to ADs and expanding our understanding of the pathomechanisms underlying ADs. Epigenetics is the study of how changes in the environment and behavior can have an impact on gene function. Indeed, researchers have found that epigenetic mechanisms can affect how genes are activated or inactivated, as well as whether they are expressed. Such mechanisms may also affect how ADs form and are protected. That is, the bulk of pharmacological trials evaluating epigenetic treatments for the treatment of ADs have used histone deacetylase inhibitors (HDACi), yielding promising outcomes in both preclinical and clinical studies. This review will provide an outline of how epigenetic pathways can be used to treat ADs or lessen their risk. It will also present the findings from preclinical and clinical trials that are currently available on the use of epigenetic drugs to treat ADs.

Epigenetic regulation in major depression and other stress-related disorders: molecular mechanisms, clinical relevance and therapeutic potential

Major depressive disorder (MDD) is a chronic, generally episodic and debilitating disease that affects an estimated 300 million people worldwide, but its pathogenesis is poorly understood. The heritability estimate of MDD is 30-40%, suggesting that genetics alone do not account for most of the risk of major depression. Another factor known to associate with MDD involves environmental stressors such as childhood adversity and recent life stress. Recent studies have emerged to show that the biological impact of environmental factors in MDD and other stress-related disorders is mediated by a variety of epigenetic modifications. These epigenetic modification alterations contribute to abnormal neuroendocrine responses, neuroplasticity impairment, neurotransmission and neuroglia dysfunction, which are involved in the pathophysiology of MDD. Furthermore, epigenetic marks have been associated with the diagnosis and treatment of MDD. The evaluation of epigenetic modifications holds promise for further understanding of the heterogeneous etiology and complex phenotypes of MDD, and may identify new therapeutic targets. Here, we review preclinical and clinical epigenetic findings, including DNA methylation, histone modification, noncoding RNA, RNA modification, and chromatin remodeling factor in MDD. In addition, we elaborate on the contribution of these epigenetic mechanisms to the pathological trait variability in depression and discuss how such mechanisms can be exploited for therapeutic purposes.

Narrative Review of the Complex Interaction between Pain and Trauma in Children: A Focus on Biological Memory, Preclinical Data, and Epigenetic Processes

The incidence and collective impact of early adverse experiences, trauma, and pain continue to increase. This underscores the urgent need for translational efforts between clinical and preclinical research to better understand the underlying mechanisms and develop effective therapeutic approaches. As our understanding of these issues improves from studies in children and adolescents, we can create more precise preclinical models and ultimately translate our findings back to clinical practice. A multidisciplinary approach is essential for addressing the complex and wide-ranging effects of these experiences on individuals and society. This narrative review aims to (1) define pain and trauma experiences in childhood and adolescents, (2) discuss the relationship between pain and trauma, (3) consider the role of biological memory, (4) decipher the relationship between pain and trauma using preclinical data, and (5) examine the role of the environment by introducing the importance of epigenetic processes. The ultimate scope is to better understand the wide-ranging effects of trauma, abuse, and chronic pain on children and adolescents, how they occur, and how to prevent or mitigate their effects and develop effective treatment strategies that address both the underlying causes and the associated physiological and psychological effects.

Chronic rapid eye movement sleep restriction during juvenility has long-term effects on anxiety-like behaviour and neurotransmission of male Wistar rats

Modernity imposes a toll on the sleep time of young population, with concomitant increase in symptoms of anxiety and depression. Whether there is a causal relationship between these events are only now being experimentally tested in humans and rodents. In a previous study, we showed that chronic sleep deprivation in juvenile-adolescent male rats led to increased anxiety-like behaviour and changes in noradrenaline and serotonin in the amygdala and hippocampus. In the present study we investigated whether early chronic sleep restriction affects emotional behaviour, stress response and neurochemistry in adulthood. From 21 to 42 days of age, Wistar male rats were submitted to sleep restriction by the multiple platform method or allowed to sleep freely. Forty-five days after this period, rats were tested in the elevated plus maze (EPM) and blood samples were collected from non-tested rats or 30 and 60 min after the EPM for determination of plasma corticosterone levels. Levels of monoamines were determined in the frontal cortex, hippocampus, amygdala and hypothalamus 60 min after the EPM. Sleep restriction resulted in increased anxiety-like behaviour, decreased noradrenaline levels in the amygdala and dopamine levels in the ventral hippocampus. Anxiety index was positively correlated with increased serotonin metabolism in the frontal cortex and greater dopamine metabolism in the ventral hippocampus, and negatively correlated with dopamine levels in the ventral hippocampus. These results suggest that sleep restriction in juvenility and adolescence induces persistent changes in emotional behaviour in adult male rats and that levels of anxiety are correlated with increased serotonin and dopamine metabolism in specific brain areas.

Paternal alcohol exposure reduces acquisition of operant alcohol self-administration and affects Bdnf DNA methylation in male and female offspring

Familial transmission of alcohol use disorder reflects genetic and environmental factors. Paternal alcohol exposure may affect rodent offspring via epigenetic modifications transmitted through the male germ line. While such exposure alters alcohol sensitivity in mouse offspring, no studies examined if it impacts the development of operant alcohol self-administration in rats. We exposed male (sires) Wistar rats to chronic intermittent ethanol in vapour chambers (16 h/day; 5 days/week) or to air for 6 weeks. Eight weeks later, rats were mated with alcohol-naive females. Adult alcohol- and control-sired F1 offspring were assessed in acquisition of alcohol self-administration in which increasing alcohol concentrations (2.5%, 5% and 10%, v/v) were delivered after one lever press (fixed ratio 1 or FR1). Prior to alcohol sessions, rats were trained to lever press for food delivery under an FR1 schedule of reinforcement. DNA methylation levels of the brain derived neurotrophic factor (Bdnf) gene were measured in sperm, nucleus accumbens (NAc) and medial prefrontal cortex (mPFC) in sires and in offspring. Alcohol-exposed sires had lower Bdnf DNA methylation levels in NAc and greater methylation levels in mPFC. Although this pattern was not recapitulated in offspring, alcohol-sired offspring of both sexes did show aberrant Bdnf DNA methylation patterns compared to control-sired offspring. Alcohol-sired offspring self-administered less alcohol (5% and 10%) with no group differences in food responding. Results indicate that paternal alcohol exposure prior to conception protects against alcohol's initial reinforcing effects but the pattern of dysregulated Bdnf methylation in reward-related circuitry did not mimic changes seen in sires.

Epigenetics of childhood trauma: Long term sequelae and potential for treatment

Childhood trauma (CT) can have persistent effects on the brain and is one of the major risk factors for neuropsychiatric diseases in adulthood. Recent advances in the field of epigenetics suggest that epigenetic factors such as DNA methylation and histone modifications, as well as regulatory processes involving non-coding RNA are associated with the long-term sequelae of CT. This narrative review summarizes current knowledge on the epigenetic basis of CT and describes studies in animal models and human subjects examining how the epigenome and transcriptome are modified by CT in the brain. It discusses psychological and pharmacological interventions that can counteract epigenetic changes induced by CT and the need to establish longitudinal assessment after CT for developing more effective diagnostics and treatment strategies based on epigenetic targets.

The Future of Livestock Management: A Review of Real-Time Portable Sequencing Applied to Livestock

Oxford Nanopore Technologies' MinION has proven to be a valuable tool within human and microbial genetics. Its capacity to produce long reads in real time has opened up unique applications for portable sequencing. Examples include tracking the recent African swine fever outbreak in China and providing a diagnostic tool for disease in the cassava plant in Eastern Africa. Here we review the current applications of Oxford Nanopore sequencing in livestock, then focus on proposed applications in livestock agriculture for rapid diagnostics, base modification detection, reference genome assembly and genomic prediction. In particular, we propose a future application: 'crush-side genotyping' for real-time on-farm genotyping for extensive industries such as northern Australian beef production. An initial in silico experiment to assess the feasibility of crush-side genotyping demonstrated promising results. SNPs were called from simulated Nanopore data, that included the relatively high base call error rate that is characteristic of the data, and calling parameters were varied to understand the feasibility of SNP calling at low coverages in a heterozygous population. With optimised genotype calling parameters, over 85% of the 10,000 simulated SNPs were able to be correctly called with coverages as low as 6×. These results provide preliminary evidence that Oxford Nanopore sequencing has potential to be used for real-time SNP genotyping in extensive livestock operations.

The Potential Role of Epigenetic Drugs in the Treatment of Anxiety Disorders

There is increasing evidence that abnormalities in epigenetic mechanisms of gene expression contribute to the pathogenesis of anxiety disorders (ADs). This article discusses the role of epigenetic mechanisms of gene expression in the pathogenesis of ADs. It also discusses the data so far obtained from preclinical and clinical trials on the use of epigenetic drugs for treating ADs. Most drug trials investigating the use of epigenetic drugs for treating ADs have used histone deacetylase inhibitors (HDACi). HDACi are showing favorable results in both preclinical and clinical drug trials for treating ADs. However, at present the mode of action of HDACi in ADs is not clear. More work needs to be done to elucidate how epigenetic dysregulation contributes to the pathogenesis of ADs. More work also needs to be done on the mode of action of HDACi in alleviating the signs and symptoms of ADs.

Hippocampal DNA Methylation in a Mouse Model of Fetal Alcohol Spectrum Disorder That Includes Maternal Separation Stress Only Partially Explains Changes in Gene Expression

Fetal alcohol spectrum disorder (FASD) is characterized by developmental and behavioral deficits caused by maternal drinking during pregnancy. Children born with FASD often face additional stresses, including maternal separation, that add yet additional deficits. The mechanism associated with this interaction is not known. We have used a mouse model for prenatal ethanol exposure and maternal separation to demonstrate that the combination of the two treatments results in more than additive deficits. Furthermore, the behavioral deficits are associated with changes in hippocampal gene expression that persist into adulthood. What initiates and maintains these changes remains to be established and forms the focus of this report. Specifically, MeDIP-Seq was used to assess if changes in promoter DNA methylation are affected by exposure to prenatal ethanol and maternal separation including its relationship to gene expression. The novel results show that different sets of genes implicated by promoter DNA methylation are affected by both treatments independently, and a relatively unique set of genes are affected by the combination of the two treatments. Prenatal ethanol exposure leads to altered promoter DNA methylation at genes important for transcriptional regulation. Maternal separation leads to changes at genes important for histone methylation and immune response, and the combination of two treatments results in DNA methylation changes at genes important for neuronal migration and immune response. Our dual results from the same hippocampal samples suggest there is minimal complementarity between changes in promoter DNA methylation and gene expression, although genes involved tend to be critical for brain development and function. While remaining to be validated, such results argue that mechanisms beyond promoter DNA methylation must be involved in lasting gene expression alterations leading to behavioral deficits implicated in FASD. They may facilitate early and reliable diagnosis, as well as novel strategies for the amelioration of FASD-related deficits.

Prenatal Exposure to Methamphetamine: Up-Regulation of Brain Receptor Genes

Methamphetamine (METH) is a widespread illicit drug. If it is taken by pregnant women, it passes through the placenta and just as it affects the mother, it can impair the development of the offspring. The aim of our study was to identify candidates to investigate for changes in the gene expression in the specific regions of the brain associated with addiction to METH in rats. We examined the various areas of the central nervous system (striatum, hippocampus, prefrontal cortex) for signs of impairment in postnatal day 80 in experimental rats, whose mothers had been administered METH (5 mg/kg/day) during the entire gestation period. Changes in the gene expression at the mRNA level were determined by two techniques, microarray and real-time PCR. Results of two microarray trials were evaluated by LIMMA analysis. The first microarray trial detected either up-regulated or down-regulated expression of 2189 genes in the striatum; the second microarray trial detected either up-regulated or down-regulated expression of 1344 genes in the hippocampus of prenatally METH-exposed rats. We examined the expression of 10 genes using the real-time PCR technique. Differences in the gene expression were counted by the Mann–Whitney U-test. Significant changes were observed in the cocaine- and amphetamine-regulated transcript prepropeptide, tachykinin receptor 3, dopamine receptor D3 gene expression in the striatum regions, in the glucocorticoid nuclear receptor Nr3c1 gene expression in the prefrontal cortex and in the carboxylesterase 2 gene expression in the hippocampus of prenatally METH-exposed rats. The microarray technique also detected up-regulated expression of trace amine-associated receptor 7 h gene in the hippocampus of prenatally METH-exposed rats. We have identified susceptible genes; candidates for the study of an impairment related to methamphetamine addiction in the specific regions of the brain.

Early life stress and the propensity to develop addictive behaviors

There is a vast literature on effects of early life manipulations in rodents much of which is aimed at investigating the long-term consequences related to emotion and cognition in adulthood. Less is known about how these manipulations affect responses reflective of alcohol (AUD) and substance (SUD) use disorders. The purpose of this paper is to review the literature of studies that employed early life manipulations and assessed behavioral responses to psychoactive substances, specifically alcohol, opiates, and stimulants, in rodents. While the findings with alcohol are more limited and mixed, studies with opiates and stimulants show strong support for the ability of these manipulations to enhance behavioral responsivity to these substances in line with epidemiological data. Some outcomes show sex differences. The mechanisms that influence these enduring changes may reflect epigenetic alterations. Several studies support a role for altered DNA methylation (and other epigenetic mechanisms) as biological responses to early environmental insults. The chemical changes induced by DNA methylation affect transcriptional activity of DNA and thus can have a long-term impact on the individual's phenotype. Such effects are particularly robust when they occur during sensitive periods of brain development (e.g., first postnatal weeks in rodents). We review this emerging literature as it relates to the known neurobiology of AUDs and SUDs and suggest new avenues of research. Such findings will have implications for the treatment and prevention of AUDs and SUDs and could provide insight into factors that support resiliency.

Pharmacological Manipulation of DNA Methylation in Adult Female Rats Normalizes Behavioral Consequences of Early-Life Maltreatment

Exposure to adversity early in development alters brain and behavioral trajectories. Data continue to accumulate that epigenetic mechanisms are a mediating factor between early-life adversity and adult behavioral phenotypes. Previous work from our laboratory has shown that female Long-Evans rats exposed to maltreatment during infancy display both aberrant forced swim behavior and patterns of brain DNA methylation in adulthood. Therefore, we examined the possibility of rescuing the aberrant forced swim behavior in maltreated-adult females by administering an epigenome-modifying drug (zebularine) at a dose previously shown to normalize DNA methylation. We found that zebularine normalized behavior in the forced swim test in maltreated females such that they performed at the levels of controls (females that had been exposed to only nurturing care during infancy). These data help link DNA methylation to an adult phenotype in our maltreatment model, and more broadly provide additional evidence that non-targeted epigenetic manipulations can change behavior associated with early-life adversity.

Epigenetic mechanisms in alcohol- and adversity-induced developmental origins of neurobehavioral functioning

The long-term effects of developmental alcohol and stress exposure are well documented in both humans and non-human animal models. Damage to the brain and attendant life-long impairments in cognition and increased risk for psychiatric disorders are debilitating consequences of developmental exposure to alcohol and/or psychological stress. Here we discuss evidence for a role of epigenetic mechanisms in mediating these consequences. While we highlight some of the common ways in which stress or alcohol impact the epigenome, we point out that little is understood of the epigenome's response to experiencing both stress and alcohol exposure, though stress is a contributing factor as to why women drink during pregnancy. Advancing our understanding of this relationship is of critical concern not just for the health and well-being of individuals directly exposed to these teratogens, but for generations to come.

Epigenetic marks are modulated by gender and time of the day in the hippocampi of adolescent rats: A preliminary study

Although the involvement of gender in epigenetic machinery in peripheral tissues during the neonatal period has been suggested, the gender-related epigenetic profile of brain areas during the adolescent period is rarely exploited. Furthermore, the influence of time of day on hippocampal acetylation marks has been demonstrated in young adult and aged rats; however, there are no studies reporting epigenetic changes in the adolescent period. Therefore, this study aimed to investigate the effects of gender on hippocampal DNA methyltransferase 1 content and histone deacetylase (HDAC) activity of adolescent rats at different time points, specifically early morning and afternoon. Both epigenetic markers increased significantly in the hippocampi of female rats compared to the male group, an indicator of reduced transcriptional activity. In addition, HDAC activity during the early morning was higher compared to afternoon groups in both male and female rats, while DNA methyltransferase 1 content was not altered by the time of day. Our findings demonstrate that hippocampal DNA methylation and histone acetylation status can be influenced by gender during the adolescent period, while the time of the day impacts HDAC activity.

Dynamic stress-related epigenetic regulation of the glucocorticoid receptor gene promoter during early development: The role of child maltreatment

Epigenetics processes may play a vital role in the biological embedding of early environmental adversity and the development of psychopathology. Accumulating evidence suggests that maltreatment is linked to methylation of the glucocorticoid receptor gene, nuclear receptor subfamily 3, group C, member 1 (NR3C1), which is a key regulator of the hypothalamus-pituitary-adrenal axis. However, prior work has been exclusively cross-sectional, greatly constraining our understanding of stress-related epigenetic processes over time. In the current study, we examined the effect of maltreatment and other adversity on change in NR3C1 methylation among at-risk preschoolers to begin to characterize within-child epigenetic changes during this sensitive developmental period. Participants were 260 preschoolers (3-5 years old, 53.8% female), including 51.5% with moderate to severe maltreatment in the past 6 months. Child protection records, semistructured interviews, and parent reports were used to assess child stress exposure. Methylation of exons 1D and 1F of NR3C1 via saliva DNA were measured at two time points approximately 6 months apart. Results indicate that maltreated children evidence higher baseline levels of NR3C1 methylation, significant decreases in methylation over time, and then at follow-up, lower levels of methylation, relative to nonmaltreated preschoolers. Findings from the current study highlight the complex nature of stress-related epigenetic processes during early development.

Transgenerational attenuation of opioid self-administration as a consequence of adolescent morphine exposure

The United States is in the midst of an opiate epidemic, with abuse of prescription and illegal opioids increasing steadily over the past decade. While it is clear that there is a genetic component to opioid addiction, there is a significant portion of heritability that cannot be explained by genetics alone. The current study was designed to test the hypothesis that maternal exposure to opioids prior to pregnancy alters abuse liability in subsequent generations. Female adolescent Sprague Dawley rats were administered morphine at increasing doses (5-25 mg/kg, s.c.) or saline for 10 days (P30-39). During adulthood, animals were bred with drug-naïve colony males. Male and female adult offspring (F1 animals) were tested for morphine self-administration acquisition, progressive ratio, extinction, and reinstatement at three doses of morphine (0.25, 0.75, 1.25 mg/kg/infusion). Grandoffspring (F2 animals, from the maternal line) were also examined. Additionally, gene expression changes within the nucleus accumbens were examined with RNA deep sequencing (PacBio) and qPCR. There were dose- and sex-dependent effects on all phases of the self-administration paradigm that indicate decreased morphine reinforcement and attenuated relapse-like behavior. Additionally, genes related to synaptic plasticity, as well as myelin basic protein (MBP), were dysregulated. Some, but not all, effects persisted into the subsequent (F2) generation. The results demonstrate that even limited opioid exposure during adolescence can have lasting effects across multiple generations, which has implications for mechanisms of the transmission of drug abuse liability in humans.

Methylation of the Glucocorticoid Receptor Gene Promoter in Preschoolers: Links With Internalizing Behavior Problems

Accumulating evidence suggests that early adversity is linked to methylation of the glucocorticoid receptor (GR) gene, NR3C1, which is a key regulator of the hypothalamic-pituitary-adrenal axis. Yet no prior work has considered the contribution of methylation of NR3C1 to emerging behavior problems and psychopathology in childhood. This study examined the links between methylation of NR3C1 and behavior problems in preschoolers. Data were drawn from a sample of preschoolers with early adversity (n = 171). Children ranged in age from 3 to 5 years, were racially and ethnically diverse, and nearly all qualified for public assistance. Seventy-one children had child welfare documentation of moderate to severe maltreatment in the past 6 months. Structured record review and interviews in the home were used to assess early adversity. Parents reported on child internalizing and externalizing behavior problems. Methylation of NR3C1 at exons 1D , 1F , and 1H were measured via sodium bisulfite pyrosequencing from saliva DNA. Methylation of NR3C1 at exons 1D and 1F was positively associated with internalizing (r = .21, p < .01 and r = .23, p < .01, respectively), but not externalizing, behavior problems. Furthermore, NR3C1 methylation mediated effects of early adversity on internalizing behavior problems. These results suggest that methylation of NR3C1 contributes to psychopathology in young children, and NR3C1 methylation from saliva DNA is salient to behavioral outcomes.

Childhood adversity and epigenetic regulation of glucocorticoid signaling genes: Associations in children and adults

Early childhood experiences have lasting effects on development, including the risk for psychiatric disorders. Research examining the biologic underpinnings of these associations has revealed the impact of childhood maltreatment on the physiologic stress response and activity of the hypothalamus-pituitary-adrenal axis. A growing body of literature supports the hypothesis that environmental exposures mediate their biological effects via epigenetic mechanisms. Methylation, which is thought to be the most stable form of epigenetic change, is a likely mechanism by which early life exposures have lasting effects. We present recent evidence related to epigenetic regulation of genes involved in hypothalamus-pituitary-adrenal axis regulation, namely, the glucocorticoid receptor gene (nuclear receptor subfamily 3, group C, member 1 [NR3C1]) and FK506 binding protein 51 gene (FKBP5), after childhood adversity and associations with risk for psychiatric disorders. Implications for the development of interventions and future research are discussed.

Insight from animal models of environmentally driven epigenetic changes in the developing and adult brain

The efforts of many neuroscientists are directed toward understanding the appreciable plasticity of the brain and behavior. In recent years, epigenetics has become a core of this focus as a prime mechanistic candidate for behavioral modifications. Animal models have been instrumental in advancing our understanding of environmentally driven changes to the epigenome in the developing and adult brain. This review focuses mainly on such discoveries driven by adverse environments along with their associated behavioral outcomes. While much of the evidence discussed focuses on epigenetics within the central nervous system, several peripheral studies in humans who have experienced significant adversity are also highlighted. As we continue to unravel the link between epigenetics and phenotype, discerning the complexity and specificity of epigenetic changes induced by environments is an important step toward understanding optimal development and how to prevent or ameliorate behavioral deficits bred by disruptive environments.

Epigenetic and gene expression changes in the adolescent brain: What have we learned from animal models?

Adolescence is defined as the gradual period of transition between childhood and adulthood that is characterized by significant brain maturation, growth spurts, sexual maturation, and heightened social interaction. Although originally believed to be a uniquely human aspect of development, rodent and non-human primates demonstrate maturational patterns that distinctly support an adolescent stage. As epigenetic processes are essential for development and differentiation, but also transpire in mature cells in response to environmental influences, they are an important aspect of adolescent brain maturation. The purpose of this review article was to examine epigenetic programming in animal models of brain maturation during adolescence. The discussion focuses on animal models to examine three main concepts; epigenetic processes involved in normal adolescent brain maturation, the influence of fetal programming on adolescent brain development and the epigenome, and finally, postnatal experiences such as exercise and drugs that modify epigenetic processes important for adolescent brain maturation. This corollary emphasizes the utility of animal models to further our understanding of complex processes such as epigenetic regulation and brain development.

Developmental stress elicits preference for methamphetamine in the spontaneously hypertensive rat model of attention-deficit/hyperactivity disorder

BACKGROUND

Developmental stress has been hypothesised to interact with genetic predisposition to increase the risk of developing substance use disorders. Here we have investigated the effects of maternal separation-induced developmental stress using a behavioural proxy of methamphetamine preference in an animal model of attention-deficit/hyperactivity disorder, the spontaneously hypertensive rat, versus Wistar Kyoto and Sprague-Dawley comparator strains.

RESULTS

Analysis of results obtained using a conditioned place preference paradigm revealed a significant strain × stress interaction with maternal separation inducing preference for the methamphetamine-associated compartment in spontaneously hypertensive rats. Maternal separation increased behavioural sensitization to the locomotor-stimulatory effects of methamphetamine in both spontaneously hypertensive and Sprague-Dawley strains but not in Wistar Kyoto rats.

CONCLUSIONS

Our findings indicate that developmental stress in a genetic rat model of attention-deficit/hyperactivity disorder may foster a vulnerability to the development of substance use disorders.

Don't worry; be informed about the epigenetics of anxiety

Epigenetic processes regulate gene expression independent of the DNA sequence and are increasingly being investigated as contributors to the development of behavioral disorders. Environmental insults, such as stress, diet, or toxin exposure, can affect epigenetic mechanisms, including chromatin remodeling, DNA methylation, and non-coding RNAs that, in turn, alter the organism's phenotype. In this review, we examine the literature, derived at both the preclinical (animal) and clinical (human) levels, on epigenetic alterations associated with anxiety disorders. Using animal models of anxiety, researchers have identified epigenetic changes in several limbic and cortical brain regions known to be involved in stress and emotion responses. Environmental manipulations have been imposed prior to conception, during prenatal or early postnatal periods, and at juvenile and adult ages. Time of perturbation differentially affects the epigenome and many changes are brain region-specific. Although some sex-dependent effects are reported in animal studies, more research employing both sexes is needed particularly given that females exhibit a disproportionate number of anxiety disorders. The human literature is in its infancy but does reveal some epigenetic associations with anxiety behaviors and disorders. In particular, effects in monoaminergic systems are seen in line with evidence from etiological and treatment research. Further, there is evidence that epigenetic changes may be inherited to affect subsequent generations. We speculate on how epigenetic processes may interact with genetic contributions to inform prevention and treatment strategies for those who are at risk for or have anxiety disorders.

Sex-dependent programming effects of prenatal glucocorticoid treatment on the developing serotonin system and stress-related behaviors in adulthood

Prenatal stress and overexposure to glucocorticoids (GC) during development may be associated with an increased susceptibility to a number of diseases in adulthood including neuropsychiatric disorders, such as depression and anxiety. In animal models, prenatal overexposure to GC results in hyper-responsiveness to stress in adulthood, and females appear to be more susceptible than males. Here, we tested the hypothesis that overexposure to GC during fetal development has sex-specific programming effects on the brain, resulting in altered behaviors in adulthood. We examined the effects of dexamethasone (DEX; a synthetic GC) during prenatal life on stress-related behaviors in adulthood and on the tryptophan hydroxylase-2 (TpH2) gene expression in the adult dorsal raphe nucleus (DRN). TpH2 is the rate-limiting enzyme for serotonin (5-HT) synthesis and has been implicated in the etiology of human affective disorders. Timed-pregnant rats were treated with DEX from gestational days 18-22. Male and female offspring were sacrificed on the day of birth (postnatal day 0; P0), P7, and in adulthood (P80-84) and brains were examined for changes in TpH2 mRNA expression. Adult animals were also tested for anxiety- and depressive- like behaviors. In adulthood, prenatal DEX increased anxiety- and depressive- like behaviors selectively in females, as measured by decreased time spent in the center of the open field and increased time spent immobile in the forced swim test, respectively. Prenatal DEX increased TpH2 mRNA selectively in the female caudal DRN at P7, whereas it decreased TpH2 mRNA selectively in the female caudal DRN in adulthood. In animals challenged with restraint stress in adulthood, TpH2 mRNA was significantly lower in rostral DRN of prenatal DEX-treated females compared to vehicle-treated females. These data demonstrated that prenatal overexposure to GC alters the development of TpH2 gene expression and these alterations correlated with lasting behavioral changes found in adult female offspring.

Environmental influences on the female epigenome and behavior

Environmental factors have long-lasting effects on brain development and behavior. One way experiences are propagated is via epigenetic modifications to the genome. Environmentally-driven epigenetic modifications show incredible brain region- and sex-specificity, and many brain regions affected are ones involved in maternal behavior. In rodent models, females are typically the primary caregiver and thus, any environmental factors that modulate the epigenotype of the mother could have consequences for her current and future offspring. Here we review evidence of the susceptibility of the female epigenome to environmental factors, with a focus on brain regions involved in maternal behavior. Accordingly, implications for interventions that target the mother's epigenome and parenting behavior are discussed.

Prenatal Choline Supplementation Diminishes Early-Life Iron Deficiency-Induced Reprogramming of Molecular Networks Associated with Behavioral Abnormalities in the Adult Rat Hippocampus

BACKGROUND

Early-life iron deficiency is a common nutrient deficiency worldwide. Maternal iron deficiency increases the risk of schizophrenia and autism in the offspring. Postnatal iron deficiency in young children results in cognitive and socioemotional abnormalities in adulthood despite iron treatment. The rat model of diet-induced fetal-neonatal iron deficiency recapitulates the observed neurobehavioral deficits.

OBJECTIVES

We sought to establish molecular underpinnings for the persistent psychopathologic effects of early-life iron deficiency by determining whether it permanently reprograms the hippocampal transcriptome. We also assessed the effects of maternal dietary choline supplementation on the offspring's hippocampal transcriptome to identify pathways through which choline mitigates the emergence of long-term cognitive deficits.

METHODS

Male rat pups were made iron deficient (ID) by providing pregnant and nursing dams an ID diet (4 g Fe/kg) from gestational day (G) 2 through postnatal day (PND) 7 and an iron-sufficient (IS) diet (200 g Fe/kg) thereafter. Control pups were provided IS diet throughout. Choline (5 g/kg) was given to half the pregnant dams in each group from G11 to G18. PND65 hippocampal transcriptomes were assayed by next generation sequencing (NGS) and analyzed with the use of knowledge-based Ingenuity Pathway Analysis. Real-time polymerase chain reaction was performed to validate a subset of altered genes.

RESULTS

Formerly ID rats had altered hippocampal expression of 619 from >10,000 gene loci sequenced by NGS, many of which map onto molecular networks implicated in psychological disorders, including anxiety, autism, and schizophrenia. There were significant interactions between iron status and prenatal choline treatment in influencing gene expression. Choline supplementation reduced the effects of iron deficiency, including those on gene networks associated with autism and schizophrenia.

CONCLUSIONS

Fetal-neonatal iron deficiency reprograms molecular networks associated with the pathogenesis of neurologic and psychological disorders in adult rats. The positive response to prenatal choline represents a potential adjunctive therapeutic supplement to the high-risk group.

Integrative Bayesian analysis of neuroimaging-genetic data with application to cocaine dependence

Neuroimaging and genetic studies provide distinct and complementary information about the structural and biological aspects of a disease. Integrating the two sources of data facilitates the investigation of the links between genetic variability and brain mechanisms among different individuals for various medical disorders. This article presents a general statistical framework for integrative Bayesian analysis of neuroimaging-genetic (iBANG) data, which is motivated by a neuroimaging-genetic study in cocaine dependence. Statistical inference necessitated the integration of spatially dependent voxel-level measurements with various patient-level genetic and demographic characteristics under an appropriate probability model to account for the multiple inherent sources of variation. Our framework uses Bayesian model averaging to integrate genetic information into the analysis of voxel-wise neuroimaging data, accounting for spatial correlations in the voxels. Using multiplicity controls based on the false discovery rate, we delineate voxels associated with genetic and demographic features that may impact diffusion as measured by fractional anisotropy (FA) obtained from DTI images. We demonstrate the benefits of accounting for model uncertainties in both model fit and prediction. Our results suggest that cocaine consumption is associated with FA reduction in most white matter regions of interest in the brain. Additionally, gene polymorphisms associated with GABAergic, serotonergic and dopaminergic neurotransmitters and receptors were associated with FA.

Effects of the Social Environment and Stress on Glucocorticoid Receptor Gene Methylation: A Systematic Review

The early-life social environment can induce stable changes that influence neurodevelopment and mental health. Research focused on early-life adversity revealed that early-life experiences have a persistent impact on gene expression and behavior through epigenetic mechanisms. The hypothalamus-pituitary-adrenal axis is sensitive to changes in the early-life environment that associate with DNA methylation of a neuron-specific exon 17 promoter of the glucocorticoid receptor (GR) (Nr3c1). Since initial findings were published in 2004, numerous reports have investigated GR gene methylation in relationship to early-life experience, parental stress, and psychopathology. We conducted a systematic review of this growing literature, which identified 40 articles (13 animal and 27 human studies) published since 2004. The majority of these examined the GR exon variant 1F in humans or the GR17 in rats, and 89% of human studies and 70% of animal studies of early-life adversity reported increased methylation at this exon variant. All the studies investigating exon 1F/17 methylation in conditions of parental stress (one animal study and seven human studies) also reported increased methylation. Studies examining psychosocial stress and psychopathology had less consistent results, with 67% of animal studies reporting increased exon 17 methylation and 17% of human studies reporting increased exon 1F methylation. We found great consistency among studies investigating early-life adversity and the effect of parental stress, even if the precise phenotype and measures of social environment adversity varied among studies. These results are encouraging and warrant further investigation to better understand correlates and characteristics of these associations.

The long-term impact of adverse caregiving environments on epigenetic modifications and telomeres

Early childhood is a sensitive period in which infant-caregiver experiences have profound effects on brain development and behavior. Clinical studies have demonstrated that infants who experience stress and adversity in the context of caregiving are at an increased risk for the development of psychiatric disorders. Animal models have helped to elucidate some molecular substrates of these risk factors, but a complete picture of the biological basis remains unknown. Studies continue to indicate that environmentally-driven epigenetic modifications may be an important mediator between adverse caregiving environments and psychopathology. Epigenetic modifications such as DNA methylation, which normally represses gene transcription, and microRNA processing, which interferes with both transcription and translation, show long-term changes throughout the brain and body following adverse caregiving. Recent evidence has also shown that telomeres (TTAGGG nucleotide repeats that cap the ends of DNA) exhibit long-term changes in the brain and in the periphery following exposure to adverse caregiving environments. Interestingly, telomeric enzymes and subtelomeric regions are subject to epigenetic modifications—a factor which may play an important role in regulating telomere length and contribute to future mental health. This review will focus on clinical and animal studies that highlight the long-term epigenetic and telomeric changes produced by adverse caregiving in early-life.

The utility of Apc-mutant rats in modeling human colon cancer

Prior to the advent of genetic engineering in the mouse, the rat was the model of choice for investigating the etiology of cancer. Now, recent advances in the manipulation of the rat genome, combined with a growing recognition of the physiological differences between mice and rats, have reignited interest in the rat as a model of human cancer. Two recently developed rat models, the polyposis in the rat colon (Pirc) and Kyoto Apc Delta (KAD) strains, each carry mutations in the intestinal-cancer-associated adenomatous polyposis coli (Apc) gene. In contrast to mouse models carrying Apc mutations, in which cancers develop mainly in the small intestine rather than in the colon and there is no gender bias, these rat models exhibit colonic predisposition and gender-specific susceptibility, as seen in human colon cancer. The rat also provides other experimental resources as a model organism that are not provided by the mouse: the structure of its chromosomes facilitates the analysis of genomic events, the size of its colon permits longitudinal analysis of tumor growth, and the size of biological samples from the animal facilitates multiplexed molecular analyses of the tumor and its host. Thus, the underlying biology and experimental resources of these rat models provide important avenues for investigation. We anticipate that advances in disease modeling in the rat will synergize with resources that are being developed in the mouse to provide a deeper understanding of human colon cancer.

Early-life stress interactions with the epigenome: potential mechanisms driving vulnerability toward psychiatric illness

Throughout the 20th century a body of literature concerning the long-lasting effects of the early environment was produced. Adverse experiences in early life, or early-life stress (ELS), is associated with a higher risk of developing various psychiatric illnesses. The mechanisms driving the complex interplay between ELS and adult phenotype has baffled many investigators for decades. Over the last decade, the new field of neuroepigenetics has emerged as one possible mechanism by which ELS can have far-reaching effects on adult phenotype, behavior, and risk for psychiatric illness. Here we review two commonly investigated epigenetic mechanisms, histone modifications and DNA methylation, and the emerging field of neuroepigenetics as they relate to ELS. We discuss the current animal literature demonstrating ELS-induced epigenetic modulation of gene expression that results in altered adult phenotypes. We also briefly discuss other areas in which neuroepigenetics has emerged as a potential mechanism underlying environmental and genetic interactions.

Litter and sex effects on maternal behavior and DNA methylation of the Nr3c1 exon 17 promoter gene in hippocampus and cerebellum

Early life events can alter gene expression through DNA methylation. The methylation status of the exon 17 promoter of the glucocorticoid receptor (Nr3c1 gene) in hippocampus associates with frequency of pup licking. Much of this work was conducted with male rats. Because dams more frequently lick male pups, this may contribute to sex differences in phenotypes through DNA methylation. Modifying litter gender composition (LGC), in which offspring of single-sex litters are compared to mixed-sex litters, alters maternal behavior. Previously, we demonstrated that LGC and sex affected pup licking times as well as anxiety and hippocampal DNA methylation of the Nr3c1 exon 17 promoter gene in adolescence. Now, we expand upon this work by examining effects in cerebellum and measuring mRNA levels. We also re-assessed DNA methylation in hippocampus using pyrosequencing and re-analyzed pup licking with the more commonly used frequency measure. Litters, culled to 8 pups on postnatal day 1 (PN1), were assigned to one of three conditions: all male (n = 10), all female (n = 12), or half of each sex (n = 20). Licking was rated on PN4, 7, and 10. On PN35, hippocampal and cerebellar samples were obtained. Single-sex males were licked the least and mixed-sex males, the most. Hippocampal Nr3c1 mRNA levels were lowest in mixed females with no LGC or Sex effects in DNA methylation. Cerebellar DNA methylation levels were lowest in mixed males with no effect on mRNA levels. Maternal pup licking associated with DNA methylation of the Nr3c1 exon 17 promoter gene in cerebellum and with hippocampal mRNA.

Decreased whole-blood global DNA methylation is related to serum hormones in anorexia nervosa adolescents

OBJECTIVES

The one-carbon metabolism, also known as methionine-homocysteine cycle, governs the dynamics of DNA methylation, epigenetically regulating gene expression, and has been reported altered in anorexia nervosa (AN) adult patients. The aim of this study consisted in assessing whole-blood DNA methylation in adolescent AN patients, assessing its significance in relationship to clinical and hormonal variables.

METHODS

Whole-blood global DNA methylation was measured as incorporation of [(3)H]dCTP following HpaII cut in 32 adolescent females affected by restrictive type AN and compared to 13 healthy controls. Homocysteine, vitamin B12 and folate plasma levels were assessed as well as fasting plasma levels of leptin and steroid hormones. Clinical variables, including severity and associate states and traits, were assessed by means of the EDI-3, CDI and STAI-Y scales.

RESULTS

We confirm that whole-blood global DNA methylation is modestly albeit significantly reduced in AN adolescents with respect to controls, correlating with plasma leptin and steroid hormone levels. Conversely, clinical traits did not correlate with the outcome variable.

CONCLUSIONS

A better definition of the epigenetic dysregulation underlying AN pathology or vulnerability might lead to develop useful markers for diagnosis, prognostic classification and tailored therapeutic interventions in these vulnerable patients since the earliest phases of their disease.

Bdnf DNA methylation modifications in the hippocampus and amygdala of male and female rats exposed to different caregiving environments outside the homecage

We have previously shown in infant rats that brief and repeated experiences with a stressed dam outside the homecage (maltreatment) alters methylation of DNA associated with the brain-derived neurotrophic factor (bdnf) gene within the developing and adult prefrontal cortex. BDNF is a key mediator of activity-dependent processes that have a profound influence on neural development and plasticity. Here we examined whether maltreatment also alters bdnf DNA methylation in two additional regions known to be prominently affected by diverse forms of early life adversity in humans- the hippocampus and amygdala. We found significant bdnf DNA methylation modifications present within the adult hippocampus (dorsal and ventral) and amygdala (central/basolateral complex). We observed that the nature of change differed between sexes, gene locus (bdnf I vs. IV), and brain region. Furthermore, a manipulation that did not produce any obvious behavior difference in infants (brief and repeated experiences with a nurturing foster dam) also had long-term effects on methylation. These data provide further empirical support of DNA methylation modifications as biological consequences of caregiving environments.

Prenatal restraint stress is associated with demethylation of corticotrophin releasing hormone (CRH) promoter and enhances CRH transcriptional responses to stress in adolescent rats

Maternal stress can disturb normal fetal neurodevelopmental progress, and lead to negative behavioral and neuroendocrine consequences for the offspring. These effects may be related to alterations in the hypothalamic-pituitary-adrenal (HPA) axis. Early life events disrupting the function of the HPA axis may be associated with epigenetic modification. This study investigated the effect of maternal stress on the methylation rate of the corticotrophin-releasing hormone (CRH) promoter and HPA axis response to acute stress in the adolescent offspring of Sprague-Dawley rats. Pregnant dams were randomly assigned to two groups: restraint stress group and normal control group. Adolescent male and female offspring were used from each group. The results showed that prenatal stress is associated with the demethylation of the CRH promoter, and leads to anxiety-like behaviors in adolescent life stages, as well as hyper-responsiveness of the HPA axis. Together, these results imply that prenatal stress alters the normal HPA function, which may be via the epigenetic mechanism.

Sex differences in DNA methylation may contribute to risk of PTSD and depression: a review of existing evidence

There are well-established sex differences in the prevalence of certain mental disorders. Work in animal models has provided us with an emerging understanding of the role that epigenetic factors play in establishing sex differences in the brain during development. Similarly, work in animal models, and a more limited but growing literature based on human studies, has demonstrated that DNA methylation (DNAm) changes occur in response to environmental stress, with some of these occurring in a sex-specific manner. In this review, we explore whether DNAm plays a role in contributing to the observed sex differences in prevalence of mental disorders in which stress contributes significantly to their etiologies, specifically posttraumatic stress disorder (PTSD) and depression. We propose that investigating sex differences in DNAm among genes known to influence brain development may help to shed light on the sexually dimorphic risk for, or resilience to, developing PTSD and depression.

Multigenerational effects of fetal-neonatal iron deficiency on hippocampal BDNF signaling

Fetal-neonatal iron deficiency induces adult learning impairments concomitant with changes in expression of key genes underlying hippocampal learning and memory in spite of neonatal iron replenishment. Notably, expression of brain-derived neurotrophic factor (BDNF), a gene critical for neuronal maturation and synaptic plasticity, is lowered both acutely and in adulthood following early-life iron deficiency. Although the mechanism behind its long-term downregulation remains unclear, epigenetic modification in BDNF, as seen in other models of early-life adversity, may play a role. Given that early iron deficiency occurs during critical periods in both hippocampal and gonadal development, we hypothesized that the iron-sufficient offspring (F2 IS) of formerly iron-deficient (F1 FID) rats would show a similar suppression of the BDNF gene as their parents. We compared hippocampal mRNA levels of BDNF and functionally related genes among F1 IS, F1 ID, and F2 IS male rats at postnatal day (P) 15 and P65 using RT-qPCR. As expected, the F1 ID group showed a downregulation of BDNF and associated genes acutely at P15 and chronically at P65. However, the F2 IS group showed an upregulation of these genes at P15, returning to control levels at P65. These results demonstrate that adverse effects of early iron deficiency on hippocampal gene expression observed in the F1 are not present in the F2 generation, suggesting differential effects of nutritionally induced epigenetic programing during the critical periods of hippocampal and gonadal development.

Sex-specific and strain-dependent effects of early life adversity on behavioral and epigenetic outcomes

Early life adversity can have a significant long-term impact with implications for the emergence of psychopathology. Disruption to mother-infant interactions is a form of early life adversity that may, in particular, have profound programing effects on the developing brain. However, despite converging evidence from human and animal studies, the precise mechanistic pathways underlying adversity-associated neurobehavioral changes have yet to be elucidated. One approach to the study of mechanism is exploration of epigenetic changes associated with early life experience. In the current study, we examined the effects of postnatal maternal separation (MS) in mice and assessed the behavioral, brain gene expression, and epigenetic effects of this manipulation in offspring. Importantly, we included two different mouse strains (C57BL/6J and Balb/cJ) and both male and female offspring to determine strain- and/or sex-associated differential response to MS. We found both strain-specific and sex-dependent effects of MS in early adolescent offspring on measures of open-field exploration, sucrose preference, and social behavior. Analyses of cortical and hippocampal mRNA levels of the glucocorticoid receptor (Nr3c1) and brain-derived neurotrophic factor (Bdnf) genes revealed decreased hippocampal Bdnf expression in maternally separated C57BL/6J females and increased cortical Bdnf expression in maternally separated male and female Balb/cJ offspring. Analyses of Nr3c1and Bdnf (IV and IX) CpG methylation indicated increased hippocampal Nr3c1 methylation in maternally separated C57BL/6J males and increased hippocampal Bdnf IX methylation in male and female maternally separated Balb/c mice. Overall, though effect sizes were modest, these findings suggest a complex interaction between early life adversity, genetic background, and sex in the determination of neurobehavioral and epigenetic outcomes that may account for differential vulnerability to later-life disorder.